1. Field of the Invention
The present invention relates to a method of identifying anomalous zones of abnormally high pore pressures in sedimentary rocks prior to drilling at a given location. More specifically, it relates to a method of identifying zones wherein the compressional and shear velocities of the earth become extremely low by using seismic data recorded at the earth""s surface.
2. Background of the Art
The most economical method of drilling a well bore is to use conventional open hole drilling techniques to drill an appropriate diameter hole with standard drilling fluid compositions. However such conventional methods are frequently not suitable where xe2x80x9cabnormallyxe2x80x9d pressured formations may be encountered by the well bore during drilling.
At a number of offshore locations, abnormally high pore pressures have been found even at relatively shallow sub-sea bottom depths (less than about 1500 meters). This could occur if a sand body containing large amounts of water is covered by silt or clay and subsequently buried. If the burial and compaction is rapid enough, xe2x80x9cdewateringxe2x80x9d and compaction of silts and clays occurs before compaction of the sand. The dewatering of clays, in particular, may result in the formation of relatively impermeable shale layers that slow down the expulsion of water from the underlying sand. The result of this is that the sand may retain high amounts of fluid and the pore pressure in the sand exceeds that which would normally be expected from hydrostatic considerations alone, i.e., the fluid pressure exceeds that which would be expected for a column of water of equivalent height. This phenomenon of overpressuring is well known to those versed in the art.
In deepwater drilling, such abnormally pressured shallow sands (also known as Shallow Water Flows or SWF) have been encountered within 4000 feet of the mudline in water depths of 1500 to 8000 feet. These sands are very high porosity units (38-45%) and are virtually unconsolidated. They exist at confining pressures from 700-6500 psi, effective stresses from 0-1000 psi, and generally show differences of only hundreds of psi between the fracture gradient and pore pressure. They are abnormally pressured and when penetrated by the drill bit, they begin to flow water at high flow rates and also begin to collapse and liquefy into the well, causing massive washouts and sometimes, complete collapse and loss of the hole.
Accurate prediction of pore pressures that may be expected along the length of a drilling well, especially exploration wells, has traditionally been a difficult industry problem. Where abnormal pressures are known to exist, or may be found unexpectedly along the length of such well bores, accurate prediction of the depth at which such pressures will be encountered may be critical to the economic success of the drilling operation. The particular problem presented in the case of Shallow Water Flows is that the abnormally-pressured interval occurs at depths where the drilling is still being done using open-hole, or riserless, methods without well-control fluids. In these circumstances, it is difficult to control flows into the wellbore and the sand flows uncontrollably with loss of fluid and sand, often causing collapse of the wellbore. In some cases, the SWF interval is cased off and cemented, but later begins to flow behind casing, undermining the cement job and causing loss of structural integrity of the casing. It is, therefore, imperative that such SWF zones be identified and characterized where possible ahead of the drillbit so that these zones can be avoided during drilling.
It is well established that the velocity of propagation of compressional and shear waves in sedimentary rocks is related to the effective stress on the rock. The effective stress on a rock at a depth is defined as the difference between the overburden stress (the weight of the overlying column of material) and the internal pore pressure in the rock. Gassmann (1951) showed that for a packing of elastic spheres, the bulk modulus of the packing is proportional to the two-thirds power of the effective stress. The bulk modulus is proportional to the square of the compressional velocity of elastic waves in a material, so that some kind of power law governs the relation between the compressional velocity and the effective stress. While a packing of elastic spheres is not a good representation of sedimentary rocks, the power law is nevertheless generally accepted to be a good empirical relation between effective stress and the compressional wave (P-wave) velocity of a rock.
A sedimentary rock is a multiphase mixture. Based on theoretical considerations, the velocity of propagation of compressional waves of a mixture of quartz (the principal component of sandstone) and water must lie between two limits called the Voight and Reuss limits. The density of such a mixture of quartz and water is simply obtained by averaging the densities of the two constituents. The upper limit of the compressional velocity is obtained by averaging the bulk modulii of the two constituents while the lower limit is obtained by averaging the compressibilities of the two constituents. The lower limit treats that mixture as one in which water is in a continuous phase and the quartz is in the form of a suspension within the water. This is a reasonably good approximation of what actually happens in the real world as sand bodies are being deposited: until the porosity drops below a value around 40%, the quartz grains are not in contact and do not bear any load. As a result of this, the compressional velocity may be given by an equation known as Wood""s equation and is very close to the velocity of sound in water, approximately 5000 ft./sec. However, using conventional seismic prospecting methods, an overpressured sand with such a high porosity is not easily detectable because the P-wave velocity is not much different from that expected for the overlying, slightly more compacted sediments.
There is a need for a method of identifying such SWF zones prior to drilling of a wellbore. Such a method should preferably make use of existing data so as to avoid the expense of additional data acquisition and processing. The present invention satisfies this need.
The present invention is a method of detecting Shallow Water Flow (SWF) sand formations having an abnormally high fluid pressure that are underneath a relatively impermeable formation having normal fluid pressures. The method uses the fact that at shallow depths, the SWF formation with abnormally high fluid pressure will show decreases in compressional wave velocity and displays a shear velocity for elastic waves that is close to zero, and is thus significantly different from the shear velocity of overlying sediments. In one aspect of the invention, the change in compressional and shear wave-velocity contrast is detected by measuring a change in the travel time and/or amplitude of seismic waves reflected from the top and base of the abnormally pressured formation. This may be done in one of two ways. The first way measures the increase in the travel time and/or amplitude of reflected compressional waves from the top and base of the anomalous formation with increasing angle of incidence, by using either conventional seismic prospecting using hydrophones or pressure detectors in water, or by ocean bottom pressure detectors. The second way measures the travel time and/or amplitude of the shear wave reflected from the top and base of the anomalous formation using motion sensors at the ocean bottom.
Another aspect of the invention uses measurements of the amplitude of reflected shear waves from a formation at some depth below the anomalous zone and relies on the fact that such a reflected shear wave cannot propagate through the anomalous zone without significant energy loss, and appears as a weak or undetectable event.